It is well known that the injection of pulverized or granular coal into the hot-air blast, which is blown through a plurality of tuyeres into a lower portion of the blast furnace, has many advantages. In particular, it increases the production capacity of the blast furnace and allows significant quantities of coke to be replaced by coal, which is far less expensive.
The injection of pulverized or granular coal is performed conventionally by means of an injection lance into the hot-air blast at a certain distance upstream from the tuyere end opening into the furnace. In other words, the coal is injected into the blowpipe or into the hot-air passage in the tuyere. The coal fed through the lance is in suspension in an inert gas and the oxidizing gas is either constituted by the hot-air blast or by pure oxygen, brought in via a separate pipe close to the nozzle end of the lance.
The combined injection of coal and oxidizing gas has been suggested e.g. in EP 0 447 908, wherein the injection is performed through a coaxial lance, wherein an outer tube is arranged surrounding an inner tube. The inner tube forms a separation wall between the oxidizing gas and the coal until both reach an outlet nozzle of the lance. Such coaxial injection lances are often referred to as oxycoal lances. In EP 0 447 908, oxidizing gas is conveyed in the outer tube and coal is conveyed in the inner tube.
Generally, the injection lance is fed through a wall portion of the blowpipe into the latter and has its outlet nozzle located preferably centrally within the flow of hot-air blast, thereby causing the hot-air blast to surround the injected coal.
In such a lance arrangement, the injection lance is subject to being damaged due to the exposure to the hot-air blast. Due to the high temperatures in the blowpipe, the lance may be deformed, which in turn may be detrimental to the correct injection of oxidizing gas and coal. Furthermore, at the tip of the lance, the conveyed coal comes into contact with the conveyed oxidizing gas, thereby creating a flame at the tip of the lance. This flame further puts at risk the tip of the lance, as well as the inner walls of the blowpipe or tuyere.
The injection of coal, in particular in combination with an oxidizing gas, becomes problematic if the gas fed through the blowpipe and tuyere is not hot-air, as is the case in conventional blast furnaces, but a reduction gas, such as is the case with blast furnaces with top gas recycling, wherein the top gas may, after treatment, be injected back into the blast furnace through the tuyere stock arrangement. It should also be noted that in the known tuyere stock arrangements, wherein the injection lance is arranged in the flow of gas through the blowpipe, the inner wall of the tuyere is at risk of being damaged by the injected coal, in particular if the tuyere nose has a reduced outlet diameter or if the coal is injected at high velocity. In case the tuyere nose has a reduced outlet diameter, the injection lance also causes a visual obstruction, at least partially preventing the blast furnace conditions to be visually monitored through the tuyere via a peep sight arranged in a bend of the blowpipe, in linear alignment with the tuyere.